3 Answers
3

Because they didn't exist originally, and the default behavior is backwards compatible. Also, because they don't exist on all unix variants, and the default behavior is compatible with other unix variants.

For many tools, because they are intended to be parseable by other tools. This is rarely the case for ls, but parsing the output of du or df is relatively common. (Mind, for df, you should use df -P when parsing.)

Because some humans prefer the 37550836 format, because when you see a bunch of such numbers, their relative size is visually clear (number of digits).

Especially 3) when I pipe the output straight through into sort -n because I'm less interested in quickly seeing the rough size of each directory and more in which subdirectory is taking up disproportionally too much space...
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ShadurNov 14 '11 at 8:41